Glycans are chains of sugar residues that have a variety of biological functional attributes within cells from providing structural content to the modification of physical and chemical properties of proteins. Glycans are also involved in extrinsic roles such as intercellular communication and immune response to pathogen infections. As a common protein post-translational modification, glycosylation occurs where a glycan is linked to specific amino acid residues in a protein. N-Linked glycosylation (or N-glycosylation) involves the attachment of a sugar chain to any Asn in the motif Asn-Xaa-Ser/Thr (where Xaa can be any amino acid except proline). The attachment occurs prior to protein folding, implying that N-glycosylation affects the tertiary structure and stability of a glycoprotein. Another distinguishing characteristic of N-glycosylation is that all N-linked glycans (or N-glycans) share a common “pentamer” core structure consisting of two N-acteylglucosamine residues (GlcNAc) and three mannose (Man) residues. O-linked glycosylation (or O-glycosylation) involves the attachment of a glycan to a Ser/Thr residue and occurs mostly after protein folding. As a result, O-glycosylation is determined not only by the local peptide sequence, but also by the global tertiary structure of proteins, and there is no known sequence motif associated with O-glycosylation sites.
Glycans exhibit enormous structural diversity through the presence of branching structures, stereomeric configurations, and flexible glycosidic bonds, with O-linked glycans (or O-glycans) showing higher structural diversity than N-glycans. In addition, a majority of human proteins have been reported to be glycosylated, making glycosylation the most prevalent and heterogeneous post-translational modifications in human proteins. Given their role in physiological and pathological responses, many glycosylation events have been connected to human diseases. Glycan recognition is fundamental to host-microbe interaction, of which the infection of influenza viruses is the most studied. Glycosylation has also been associated with various cancers where variations in glycan and glycoprotein abundance have been observed in cancer patients in comparison with healthy individuals. This association warrants the study of glycosylation to develop potential disease biomarkers, especially from human serum samples. Although computational methods based on mass spectrometry data have proven to be effective in monitoring changes in the glycome, developing such methods for the glycoproteome could be challenging, largely due to the inherent complexity in simultaneously studying glycan structures and corresponding glycosylation sites.